Any comic that isn't new is called a Back Issue. Some comic shops keep the last several months on the rack and sell them like new comics, others place the older issue in their back issue bins as soon as a newer issue comes out. Some back issues go up in value, some are "hot" (meaning demand is greater than perceived supply) and are sold at extremely elevated prices. It is very rare for a "hot" comic to hold the escalated value over the course of 6 months to 2 years. If you don't really want/need it now, you can often wait for a year or two and get it cheaper. Many issues also end up in "dollar bins" and are sold at discount prices. Ultimately, there are many different ways to get your hands on back issue comics:
Local Comic Shops - Many comic shops have back issue sections, but it seems like fewer and fewer carry back issues every year. Also, some shops tend to carry fairly recent back issues (basically stock that they ordered and failed to sell as new comics over the past several years) whereas other shops specialize in buying and selling back issues. Familiarize yourself with the comic shops in your area so you know what shops are best at selling what kinds of comics.
Conventions - Not everyone has a convention nearby but if you have a chance to attend one, many have a number of dealers with all kinds of comics, new and old. A good rule of thumb at a convention is to check out multiple dealers before making your purchase. It's always disheartening to buy a comic at dealer A and then moving down an aisle to find the same comic for less money, in better condition, or both from another dealer.
Auction Houses - There are auction houses that specialize in back issues and original art. These are great places to purchase the issues you're looking for and many of the Auction Houses are fair in grading and provide some outstanding service. Be careful of "Buyer's Premiums" that add a large % to your purchase price and high shipping/handling fees. Familiarize yourself with all the terms and conditions of any auction house you plan to use.
eBay - An excellent resource for Back Issues, Original art etc. You should set up an eBay account even if you never plan to buy or sell anything there. From your account, there are many tools to assist you in collecting information about the back issue market. You can search for particular titles, issue numbers and more. Or, you can even go right to the comics section to just browse at your leisure. There are even sub-categories under "Comics" for things like Bronze Age, Silver Age and Graphic Novels. Remember that you're relying on the seller choosing those particular categories so you may be missing out on items listed more generically whenever you use the eBay categories. Categories may also be filled with spurious items placed there to get you to look at them.
Also, with most things on e-Bay there is a certain risk involved when it comes to the condition of the item. And with back issue comics, it becomes even more important. You're version of "Fine" may not be the seller's version of "Fine". Look for quality scans of the back issue you're looking at and don't hesitate to ask the seller any questions you may have.
Estate Sales - If you can beat the local comic dealers to these there are sometimes back issues available at estate sales for great prices. Keep an eye on local newspapers & web-sites for information on what's in your area.
Flea Markets/Swap Meets - Although these can be hit or miss when it comes to finding back issues, you can sometimes come across some decent buys at a local flea market or swap meet. Be aware that people at flea markets usually have an inflated view of what their comics are actually worth and may be hazy on the concept of comic book grading and the relationship between grade and value of a comic book.
Almost any comic can be found at some on-line retailer. An advantage is availability, a drawback is that you usually can't physically inspect what you're buying before you buy it. Look for good pictures, a solid return policy, and recommendations from people you know.
Keep in mind that grading is very subjective and the most important thing for me is finding a seller who shares approximately the same grading aesthetic as you do. This is going to vary person by person and you're going to need to feel out dealers to see how you click with them and their grading. When buying cheap books (a couple of $a piece) you might decide to roll the dice and put in a small order and just see how the grading looks compared to your personal standards. When buying more expensive books it is always a good idea to ask for specific individual pictures/scans of what you are considering purchasing so you can have a look at what you'll be getting in advance from dealers you are not familiar with. Make informed decisions and you'll generally be happier with your on-line purchases.
There are a number of comic dealers and web-sites that the staff at ComicSpectrum browsed and bought from, we provide some guidance below based on our own experiences. Keep in mind that we are not typically in the market for absolute perfection in our comics and a small bump or ding on a book he pays $1 for is not going to make us upset. That said, we would be a lot more picky about a book advertised as being NM and selling for $100+. Keep in mind that it is difficult to suss out the difference between a 9.6 and 9.8 from pictures. If you are looking for high-grade books and are very picky about grade it is best to either buy them in person at shops or conventions or buy books that have been professionally graded.
The opinions offered here are broad guidance. As always, Your Mileage May Vary. Good luck!!
ComicCollectorLive- A collective of individual sellers, many with free shipping and cheap comics. Tied in with the ComicCollectorLive software (if you use that to track your collection). Even if you don't use it, you can create a want list and search sellers automatically for books you're looking for. Our experience is that some of the sellers are really good and others not so much.
Bob's personal thoughts:
Amazing Cards & Comics (WIZZARD27) - Does not match with my personal opinions on books in the VG and F conditions, personally I'd grade books he sent me scans of 1/2 to a full grade lower.
Blue Moon Comics (BlueMoon) - Accurate grading on older stuff down to VG. Beware that recent Marvel comics may have had digital codes redeemed, but they're usually really cheap, so it's a trade-off.
East Bay Comics (ebcomics) - Solid grading. I bought my Journey Into Mystery #86 in VG/F from them and was very happy with the grade.
Hall of Heroes (MoonKnight1) - Solid grading in the VG to Fine range, if anything they tend to undergrade a bit just to be safe. Got my Tales to Astonish #39 and #42 from them, both graded VG.
ComicConnect - Auction House that's not as user friendly as ComicLink. Nice selection of both CGC and unslabbed books
ComicLink - An auction house that's easy to use. A lot of CGC graded comics and minimum bid is usually half of asking price. Great to use for actual realized prices so it's a great research tool much like e-Bay. Problem is there is no Want List or saved search feature (or at least none I could find). Be aware that if you buy a "lot" of comics, ComicLink will not allow you to check out & combine shipping with other individual purchases, so you'll need to pay a separate shipping fee (around $20) for the lot. They charge a 3% Buyer's Premium on all winning auction bids.
Bob's personal thoughts - I've found some nice items on here that I won at prices below guide, as with any auction site just be careful and don't get caught up in a bidding war that causes you to pay more than you should. I've mostly bought slabbed key books that cost $80-100 or more, and even then a couple of books in the same auction so the high shipping cost is not as much as a percentage of the final cost (more like 10% instead of 50%). I would not even consider this for buying things that cost <$20 unless I was buying a large number of items to amortize the $20 and up shipping cost. Ship time is pretty slow, generally takes a couple of weeks after auction close for me to receive books.
Greg Reece Rare Comics - Mostly CGC books, but has sections for each age and has some non-slabbed books too. Accurate grading, but expect to pay a bit of a premium. Also, fairly high shipping, so not a good place to get just one cheap comic
High Grade Comics - Smaller selection but specialize Very Fine+ and Key books. Expect to pay more for the high quality condition.
Midtown Comics - Not a great selection for pre-1990 books, but they do have cheaper back issues and good prices on "current" back issues.
Mile High Comics - One of the largest selection of comics on the web. Expect to pay more for the convenience of finding the issue that you need, a common joke about them relates to their "mile high" prices. Subscribe to their mailing list and utilize their frequent sale codes which bring their "mile high" prices down to just around the high end of normal.
Bob's personal thoughts - Be careful about their home-brewed grading standards, especially on mid-grade books. It won;t happen every time, and maybe you'll get lucky and it will never happen to you, but years ago I bought a Tomb Of Dracula #10 graded as Fine from them that came with a 1/2" tear on the cover. Surely this was misgraded. When I contacted their customer service, expecting a refund I was told to have a look at their grading standards, according to their standards a 1/2" tear is allowable in Fine. Personally I'd have called it VG- at best. No refund offered, they told me condition was as advertised. It was the last time I ordered a book from them in an advertised grade lower than NM (and even a NM book could have a 1/8" tear on the cover and comply with their standards!!)
MyComicShop.com - Great selection, great prices and fair grading. Highly recommended! You can also build a want list on the site and be notified when comics you are looking for become available.
Bob's personal thoughts - Meticulous grading on books they actually take the time to grade. I never hesitate to buy something that costs more than $10 that is graded 4.0 (or even 3.0). I have bought books from them costing $100s and have been very happy with them. They are tough graders and match my personal grading standards very well. That said, a lot of their "bulk books" that are cheap don't get any specific personal attention and tend to just be given a default grade of NM. I have gotten $1 books graded NM with bumped corners, minor spine damage, scuffed covers (basic shelf wear) since they don't tend to individually bag and board the cheap books in warehouse storage. Not every book will have a problem, mind you, but it happens. When it does they will cheerfully offer a refund, but don't flip out if you get a cheap book for a buck or 2 and it doesn't look like it came directly from the printer.
Pedigree Comics - Not a huge selection, but of what they do have is high grade and CGC. If you're looking for key Silver and Golden age books this is a good place to look.
Quality Comix - Similar to Pedigree in amount of selection, but a wider range of books with plenty non-CGC. Expect to pay Overstreet guide pricing.
SuperWorld Comics - Higher prices than most, but a decent selection of older books. Not an easy site to navigate.
Things from Another World (TFAW) - A little bit of everything, great for their nick and dent sales on graphic novels that happen a couple of times a year. Join their mailing list for notifications of sales and coupon codes.
Worldwide Comics - Nice easy to use site, with fair prices. Owners are writing a book about pedigreed comics so they know their stuff.